Wireless or mobile communications networks in which a mobile terminal (UE, such as a mobile handset) communicates via a radio link to a network of base stations (eNBs) or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations. The initial deployment of systems using analogue signalling has been superseded by second generation (2G) digital systems such as Global System for Mobile communications (GSM), which typically use a radio access technology known as GSM Enhanced Data rates for GSM Evolution Radio Access Network (GERAN), combined with an improved core network.
Second generation systems have themselves been replaced by or augmented by third generation (3G) digital systems such as the Universal Mobile Telecommunications System (UMTS), which uses a Universal Terrestrial Radio Access Network (UTRAN) radio access technology and a similar core network to GSM. UMTS is specified in standards produced by 3GPP. Third generation standards provide for a greater throughput of data than is provided by second generation systems. This trend is continued with the move towards fourth generation (4G) systems.
GPP design, specify and standardise technologies for mobile (cellular) wireless communications networks. Specifically 3GPP produces a series of technical reports (TR) and technical specifications (TS) that define 3GPP technologies. The focus of 3GPP is currently the specification of standards beyond 3G, and in particular an Evolved Packet System (EPS) offering enhancements over 3G networks, including higher data rates. The set of specifications for the EPS comprises two work items: Systems Architecture Evolution (SAE, concerning the core network) and LTE concerning the air interface. The first set of EPS specifications were released as 3GPP Release 8 in December 2008. LTE uses an improved radio access technology known as Evolved UTRAN (E-UTRAN), which offers potentially greater capacity and additional features compared with previous standards. SAE provides an improved core network technology referred to as the Evolved Packet Core (EPC). Despite LTE strictly referring only to the air interface, LTE is commonly used to refer to the whole of the EPS, including by 3GPP themselves. LTE is used in this sense in the remainder of this specification, including when referring to LTE enhancements, such as LTE Advanced. LTE is an evolution of UMTS and shares certain high level components and protocols with UMTS. LTE Advanced offers still higher data rates compared to LTE and is defined by 3GPP standards releases from 3GPP Release 10 up to and including 3GPP Release 12. LTE Advanced is considered to be a 4G mobile communication system by the International Telecommunication Union (ITU).
The present invention may be implemented within a 2G/3G or LTE mobile network. An overview of an LTE network is shown in FIG. 1. The LTE system comprises three high level components: at least one UE 102, the E-UTRAN 104 and the EPC 106. The EPC 106 communicates with Packet Data Networks (PDNs) and servers 108 in the outside world. FIG. 1 shows the key component parts of the EPC 106. It will be appreciated that FIG. 1 is a simplification and a typical implementation of LTE will include further components. In FIG. 1 interfaces between different parts of the LTE system are shown. The double ended arrow indicates the air interface between the UE 102 and the E-UTRAN 104. For the remaining interfaces media is represented by solid lines and signalling is represented by dashed lines.
The E-UTRAN 104 comprises a single type of component: an eNB which is responsible for handling radio communications between the UE 102 and the EPC 106 across the air interface. An eNB controls UEs 102 in one or more cell. LTE is a cellular system in which the eNBs provide coverage over one or more cells. Typically there is a plurality of eNBs within an LTE system. In general, a UE in LTE communicates with one eNB through one cell at a time.
Key components of the EPC 106 are shown in FIG. 1. It will be appreciated that in an LTE network there may be more than one of each component according to the number of UEs 102, the geographical area of the network and the volume of data to be transported across the network. Data traffic is passed between each eNB and a corresponding Serving Gateway (S-GW) 110 which routes data between the eNB and a PDN Gateway (P-GW) 112. The P-GW 112 is responsible for connecting a UE to one or more servers or PDNs 108 in the outside world. The Mobility Management Entity (MME) 114 controls the high-level operation of the UE 102 through signalling messages exchanged with the UE 102 through the E-UTRAN 104. Each UE is registered with a single MME. There is no direct signalling pathway between the MME 114 and the UE 102 (communication with the UE 102 being across the air interface via the E-UTRAN 104 and thus an eNB). Signalling messages between the MME 114 and the UE 102 are provided by the Non-access stratum (NAS) layer and comprise EPS Mobility Management (EMM) and EPS Session Management (ESM) protocol messages. The EMM protocol supports the mobility of a UE such as informing the network of its location, providing control of security and providing connection management services to the session management sublayer, for example in order to create a signalling connection for the UE to send data to the network. The ESM protocol provides procedures for the handling of EPS bearer contexts which are signalling contexts provided between the UE and the P-GW to control the flow of data from the UE to the outside world. The MME 114 exchanges signalling traffic with the S-GW 110 to assist with routing data traffic. The MME 114 also communicates with a Home Subscriber Server (HSS) 116 which stores information about users registered with the network.
Key components of the UE 102 and an eNB 202 which forms part of the E-UTRAN are shown in FIG. 2. The UE includes at least a receiver 204 for receiving radio communications from the eNB over the air interface, a transmitter 206 for transmitting radio communications to the eNB over the air interface and a controller 208 which is configured to control the receiver and transmitter to perform the reception and transmission of radio communications to and from the eNB respectively. Although the UE is illustrated as comprising only a receiver, transmitter and controller, it may also include any number of other components such as memory and a processor for example. The eNB includes at least a receiver 210 for receiving radio communications from the UE over the air interface, a transmitter 212 for transmitting radio communications to the UE over the air interface and a controller 214 which is configured to control the receiver and transmitter to perform the reception and transmission of radio communications to and from the UE respectively, where signalling messages between the UE 102 and the eNB 202 are provided by the Access Stratum (AS) layer. As for the UE, the eNB may also include any number of other components such as a processor and memory for example as well as components required to communicate with the EPC. Also, for both the UE 102 and the eNB 202 although separate transmitters and receivers are shown, they may be combined to form a transceiver at the UE and a transceiver at the eNB. The eNBs which form the E-UTRAN may also include an X2 interface for communicating between one another for purposes such as handover for example.
When a UE is first switched on or first enters a coverage area of the E-UTRAN, the UE is required to attach to a network. Conventionally, a UE in such a scenario will attempt to attach to network to which it has a subscription, or more specifically a network specified by information stored on the USIM of the UE. Once a network corresponding to the USIM information has been detected, the UE will request a connection to the network via the serving eNB in order to transmit an attach request to an MME of the network.
Mobile communications networks such as that illustrated in FIG. 1 are also referred to as Public Land Mobile Networks (PLMN) and different PLMNs are identified via identifiers which include a Mobile Country Code (MMC) and a Mobile Network Code (MNC). Networks operated by different operators will have different identifiers as well as networks which operate in accordance with different standards such as GSM, 3G and LTE and those in different countries. Accordingly, when a UE is first switched on it will firstly detect available networks and if a network PLMN identity which corresponds to its “home network” identity is detected, it will transmit an attach request to the network.